Hair cells, the sensory cells of the inner ear, transduce auditory and vestibular stimuli with their mechanically sensitive hair bundles. Hair bundles contain a small subset of all proteins expressed in hair cells, presumably reflecting bundles' specialized function. Identification of these bundle proteins, difficult because of the small numbers of bundles in any auditory or vestibular organ, will lead to new hypotheses for the assembly, structure, maintenance, and function of hair bundles. Recently, the sensitivity of mass spectrometry methods has increased sufficiently to contemplate large-scale sequencing of the proteins making up the hair bundle (the hair-bundle "proteome"). In our first set of proposed experiments, we will use the isotope-coded affinity tag (ICAT) method to differentially label proteins expressed in hair bundles (comparing to hair- and supporting-cell somas, as a control), then use automated mass-spectrometry methods to identify bundle proteins of high and moderate abundance. In our second set of experiments, we will determine which hair-bundle proteins change in bundle expression when the molecular motor myosin-7a, crucial for human and mouse auditory function, is missing. These experiments will allow us to identify the major proteins of the bundle and allow us to focus on proteins functionally important for bundle function.